Precipitation Hardening

OCTOBER 14, 2008

Precipitation Hardening A brief report based on literature Jyoti Swaroop Repaka, Shabolu Suryakanth, P N V Prudhvi Teja

Precipitation Hardening

Precipitation Hardening Precipitation Hardening is a heat treatment technique often employed in alloys to improve mechanical properties such as strength and hardness.

Motivation Almost every product of industry contains metal parts or is manufactured by machines made of metal parts. Metals and their alloys are now a very important constituent of our everyday lives. One of the most simple and useful properties of metals is that of hardness. Solid solution hardening , strain hardening, and precipitation hardening are few important strengthening mechanisms in alloys. Precipitation hardening is used as a strengthening mechanism in most aluminium alloys and some stainless steels.

History The fundamental under-standing and basis for this technique was established in early work at the U.S. Bureau of Standards on an alloy known as Duralumin. (Duralumin is an aluminium alloy containing copper and magnesium with small amounts of iron and silicon). In an attempt to understand the dramatic strengthening of this alloy, Paul D. Merica and his coworkers studied both the effect of various heat treatments on the hardness of the alloy and the influence of chemical composition on the hardness. Among the most significant of their findings was the observation that the solubility of CuAl2 in aluminium increased with increasing temperature. Although the specific phases responsible for the hardening turned out to be too small to be observed directly, optical examination of the microstructures provided an identification of several of the other phases that were present. The principle features of their theory are as follows: 1. Age-hardening is possible because of the solubility-temperature relation of the hardening constituent in aluminium. 2. The hardening constituent is CuAl2. 3. Hardening is caused by precipitation of the constituent in some form other than that of atomic dispersion, and probably in fine molecular, colloidal or crystalline form. 4. The hardening effect of CuAl2 in aluminium was deemed to be related to its particle size.

Literature Survey Precipitation hardening, also called age hardening, is a heat treatment technique used to strengthen malleable materials. It relies on changes in solid solubility with temperature to produce fine particles of an impurity phase, which impede the movement of dislocations, or defects in a crystal's lattice. Since dislocations are often the dominant carriers of plasticity, this serves to harden

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Precipitation Hardening the material. Unlike ordinary tempering, alloys must be kept at elevated temperature for hours to allow precipitation to take place. This time delay is called ageing. The primary species of precipitation strengthening are second phase particles. These particles impede the movement of dislocations throughout the lattice. You can determine whether or not second phase particles will precipitate into solution from the solidus line on the phase diagram for the particles. Physically, this strengthening effect can be attributed both to size and modulus effects, and to interfacial or surface energy. The presence of second phase particles often causes lattice distortions. These lattice distortions result when the precipitate particles differ in size from the host atoms. Smaller precipitate particles in a host lattice leads to a tensile stress, whereas larger precipitate particles leads to a compressive stress. Dislocation defects also create a stress field. Above the dislocation there is a compressive stress and below there is a tensile stress. Consequently, there is a negative interaction energy between a dislocation and a precipitate that each respectively cause a compressive and a tensile stress or vice versa. In other words, the dislocation will be attracted to the precipitate. In addition, there is a positive interaction energy between a dislocation and a precipitate that have the same type of stress field. This means that the dislocation will be repulsed by the precipitate. Precipitate particles also serve by locally changing the stiffness of a material. Dislocations are repulsed by regions of higher stiffness. Conversely, if the precipitate causes the material to be locally more compliant, then the dislocation will be attracted to that region. Furthermore, a dislocation may cut through a precipitate particle. This interaction causes an increase in the surface area of the particle. The area created is

where, r is the radius of the particle and b is the magnitude of the burgers vector. The resulting increase in surface energy is

where

is the surface energy. The dislocation can also bow around a precipitate particle. There are 2 equations governing the mechanism of precipitation hardening:

where τ is material strength, r is the second phase particle radius, γ is the surface energy, b is the magnitude of the Burgers vector, and L is the spacing between pinning points.

And where τ is the material strength, G is the shear modulus, b is the magnitude of the Burgers vector, L is the distance between pinning points, and r is the second phase particle radius.(note: Burger’s

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Precipitation Hardening vector is a vector that represents the magnitude and direction of the lattice distortion of dislocation in a crystal lattice)

Examples and Discussion Precipitation hardening aluminium alloys Precipitation hardening is often employed in Aluminium alloys to improve the mechanical properties, such as strength and hardness. This change in properties is the result of the formation of finely dispersed second phase particles in the alloy. These particles induce lattice strain in the Aluminium matrix that restricts dislocation flow. The material is then heated again to a temperature below the solvus temperature and held for some amount of time. This temperature is known as the age hardening temperature. The purpose of heating the alloy again to decrease the time needed to age the material. Examples include 2000-series aluminium alloys (like 2024 and 2019), 6000-series aluminium alloys and 7000-series aluminium alloys (like 7075 and 7475).

Precipitation hardening stainless steels Precipitation hardening stainless steels are chromium and nickel containing steels that provide an optimum combination of the properties of martensitic and austenitic grades. Like martensitic grades, they are known for their ability to gain high strength through heat treatment and they also have the corrosion resistance of austenitic stainless steels. The high tensile strengths of precipitation hardening stainless steels come after a heat treatment process that leads to precipitation hardening of a martensitic or austenitic matrix. Hardening is

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Precipitation Hardening achieved through the addition of one or more of the elements Copper, Aluminium, Titanium, Niobium, and Molybdenum. The most well known precipitation hardening steel is 17-4 PH. The name comes from the additions 17% Chromium and 4% Nickel. It also contains 4% Copper and 0.3% Niobium. 17-4 PH is also known as stainless steels grade 630. The advantage of precipitation hardening steels is that they can be supplied in a “solution treated” condition, which is readily machineable. After machining or another fabrication method, a single, low temperature heat treatment can be applied to increase the strength of the steel. This is known as ageing or age-hardening. As it is carried out at low temperature, the component undergoes no distortion. Discussion This technique exploits the phenomenon of super saturation, and involves careful balancing of the driving force for precipitation and the thermal activation energy available for both desirable and undesirable processes. Nucleation occurs at a relatively high temperature (often just below the solubility limit) so that the kinetic barrier of surface energy can be more easily overcome and the maximum number of precipitate particles can form. These particles are then allowed to grow at lower temperature in a process called aging. This is carried out under conditions of low solubility so that thermodynamics drive a greater total volume of precipitate formation. Diffusion's exponential dependence upon temperature makes precipitation strengthening, like all heat treatments, a fairly delicate process. Too little diffusion (under aging), and the particles will be too small to impede dislocations effectively; too much (over aging), and they will be too large and dispersed to interact with the majority of dislocations.

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Precipitation Hardening A large number of other constituents may be unintentional, but benign, or may be added for other purposes such as grain refinement or corrosion resistance. In some cases, such as many aluminium alloys, an increase in strength is achieved at the expense of corrosion resistance. The addition of large amounts of nickel and chromium needed for corrosion resistance in stainless steels means that traditional hardening and tempering methods are not effective. However, precipitates of chromium, copper or other elements can strengthen the steel by similar amounts to hardening and tempering. The strength can be tailored by adjusting the precipitation temperature, with lower temperatures resulting in higher strengths. Many alloy systems allow the aging temperature to be adjusted. For instance, some aluminium alloys used to make rivets for aircraft construction are kept in dry ice from their initial heat treatment until they are installed in the structure. After this type of rivet is deformed into its final shape, aging occurs at room temperature and increases its strength, locking the structure together. Higher aging temperatures would risk over-aging other parts of the structure, and require expensive post-assembly heat treatment.

References i.

ii. iii. iv.

P. D. Merica, R. G. Waltenberg, and H. Scott, Heat Treatment of Duralumin, Bull. Am. Inst. Min. Metall. Eng. 150, 913-949 (1919); also P. D. Merica, R. G. Waltenberg, and H. Scott, Heat-Treatment of Duralumin, Sci. Pap. Bur. Stand. 15, 271-316 (1919). Askeland, Donald R., The Science and Engineering of Materials, Boston: PWS Publishing, 1994. W.D. Callister. Fundamentals of Materials Science and Engineering, Wiley & Sons. http://www.azom.com , aalco.

Details of the assignment Participants Roll Number 07010041 07010042 07010044

Name Shabolu Suryakanth Repaka Jyoti Swaroop Puvvada N V Prudhvi Teja

Course details MM 207 – Engineering Metallurgy

14 October 2008 Indian Institute of Technology Bombay

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Department Mechanical engineering Mechanical engineering Mechanical engineering